In FIG. 2 of WO 01/036235 (PCT Japanese Translation Patent Publication No. 2003-525797), a structure is described that has a retractor mounted on the floor of a vehicle and allows the tip side of an inflatable lap belt to be retracted by the retractor. In this prior example, an inflator is mounted on the vehicle floor and gas is introduced into the lap belt through a special structure, so called a plenum.    [Patent Document 1] PCT Japanese Translation Patent Publication No. 2003-525797
In the PCT Japanese Translation Patent Publication No. 2003-525797, as shown in FIG. 2 thereof, an inflator and a gas supply tube connecting the inflator to a plenum (the tip side of a lap belt) are exposed in a vehicle cabin. There is therefore a risk that the inflator is directly sprayed with water or covered with dust, or that the inflator and/or the gas supply tube are damaged due to rubbing or the like. Furthermore, the appearance of the interior of the vehicle cabin is defiled.